METAMORPHOSIS GENERALLY. 51 



portionately large head of the dragon flies, which are 

 the types of the Neu7-optera, find their counterparts in 

 the whales and other Cetacea; both also are eminently 

 aquatic. We shall not, for the present, pursue the subject 

 further, since enough has now been said on the exter- 

 nal * analogies of the Ptilota, to establish their relations 

 to the vertebrate groups. How far these will be mani- 

 fested among the apterous class will appear in the sequel. 

 (47.) Metamorphosis, as we have already shown, 

 may be termed the chief philosophic distinction of the 

 annulose animals ; although, as it is a peculiarity which 

 is neither permanent, nor can be detected at all times by 

 the eye, it is not so convenient for popular use as those 

 characters drawn from the external structure of the 

 body. Now, this metamorphosis, or change of form, is 

 produced by exactly the same general process as that 

 which is usually termed ecdysis, or moulting ; that is, 

 the external skin or covering, at certain seasons or 

 periods of growth, is thrown off, and the animal appears 

 in a new one, which has been forming beneath. In 

 both cases, a change in the outward covering is effected ; 

 but here the similarity ceases. When a quadruped, at 

 the approach of summer, casts off by degrees the thick 

 coat of hair or wool which Nature had given it to resist 

 the cold of winter, the change, to a superficial observer, 

 is scarcely perceptible ; the new hairs are of the same 

 colour, and nearly of the same texture, as the old ; still 

 less do we see any change in the outward form : the 

 same may be said of the generality of birds ; although, 

 in this class, nature evidently proceeds a step further in 

 her transformations. Among the water fowl, for in- 

 stance, particularly in the wading order, we see the 

 summer plumage much more gay than that of the 

 •winter ; and, in many cases, so very different, that the 

 same species has not unfrequently been described under 

 two different names. It is a most singular fact, also, 

 that nearly all the families and genera which represent 



* By external analogies, we mean such as relate to groups out of tte 

 circle ot the Annulosa. 



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